Odd brake issue - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72LB · 12-01-2018, 09:47 PM

I just did a complete brake overhaul to my 69, lines , disc conversion , booster/MC even a new rearend. Bled the MC and the whole truck , all is good there. The issue is with the truck off I have a hard pedal but when I start the truck brakes go to the floor. I've checked the check valve and it seems ok, I get a small poof out of the booster when I take it out. It seems I'm at a wall with this one, anyone have any suggestions?

Andy4639 · 12-02-2018, 01:02 PM

What proportioning valve do you have? Did you use the little plastic centering tool for bleeding the brakes? Did you bench bleed the MC before you installed it? How about the rod length and how do you have your pedal setup?

I see your in Inman SC are you a member of our club?

Upstate GM trucks?

72LB · 12-02-2018, 02:06 PM

The prop valve is CPP and the booster/ MC is summit. Bench bled the MC and I used the tool and bled the truck 3 times, nothing but fluid coming out.The rod length is set up similar to my old one from measurements it took of all of it when I removed the old.

Yep I'm a club member.
V
V
V

Andy4639 · 12-02-2018, 02:11 PM

Is this you Harry?

72LB · 12-02-2018, 03:19 PM

yep.

Andy4639 · 12-02-2018, 09:15 PM

How are the rear shoes. Are they tight on the hub when you install it? If not you need to adjust them to just barely rub to make them work.This needs to be done before bleeding.

72LB · 12-02-2018, 10:18 PM

Did that.

Andy4639 · 12-03-2018, 07:03 AM

Well as we spoke on the club page I would get a new prop, valve and start over.

68gmsee · 12-03-2018, 11:31 AM

In case you haven't fixed the problem: I had a similar problem on my truck and it turned out to be new defective brake master cylinder -actually two from same parts store. I finally got my money back and got a different brand. The difference between you're problem an mine is that my pedal would go down to the floor but not really fast. Turns out the master cylinder piston was allowing fluid to go past it.

I'm sure you already know this but for others that may not know, it's very important that you bench bleed the master cylinder according to the directions.

72LB · 12-03-2018, 08:33 PM

Andy from what I have found a bad prop valve won't do what this one is doing.

68gmsee from what I have found it can be one of two problems, a bad booster or master. Changed out the booster this afternoon and no change so MC is next tomorrow.

68gmsee · 12-04-2018, 04:20 PM

Hope the mc does it for you. If possible, try a different brand.

Other than lots of air in the lines and or really bad leak, there's not much else. Keep us posted on the fix.

RichardJ · 12-04-2018, 05:28 PM

>>with the truck off I have a hard pedal<<
That pretty much rules out bleeding, shoe adj or Metering valve or Proportion valve, unless you had to pump the pedal to get that hard pedal.
>> but when I start the truck brakes go to the floor.<<
As it should, to a point.

A correct size and operating booster should allow three strokes of the pedal should the engine die and loss of manifold vacuum.

With the engine OFF, brake pedal depressed enough times to release all vacuum - while holding pedal down, start the engine. The pedal SHOULD drop.

If the pedal goes all the way to the floor, the booster is working, the MC is working. but you DO have a problem.

It sounds like one of two possible problems. Either the MC piston diameter is too small or the pedal-to-booster rod is mounted too high on the pedal arm.

12-01-2018, 09:47 PM	#1
72LB Registered User Join Date: May 2017 Location: Inman/Boiling Springs, SC Posts: 420	Odd brake issue I just did a complete brake overhaul to my 69, lines , disc conversion , booster/MC even a new rearend. Bled the MC and the whole truck , all is good there. The issue is with the truck off I have a hard pedal but when I start the truck brakes go to the floor. I've checked the check valve and it seems ok, I get a small poof out of the booster when I take it out. It seems I'm at a wall with this one, anyone have any suggestions? __________________ 1985 C10 shortbed, current project. Member of the Upstate SC GM Truck Club.

12-02-2018, 01:02 PM	#2
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Odd brake issue What proportioning valve do you have? Did you use the little plastic centering tool for bleeding the brakes? Did you bench bleed the MC before you installed it? How about the rod length and how do you have your pedal setup? I see your in Inman SC are you a member of our club? Upstate GM trucks? __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

12-02-2018, 02:06 PM	#3
72LB Registered User Join Date: May 2017 Location: Inman/Boiling Springs, SC Posts: 420	Re: Odd brake issue The prop valve is CPP and the booster/ MC is summit. Bench bled the MC and I used the tool and bled the truck 3 times, nothing but fluid coming out.The rod length is set up similar to my old one from measurements it took of all of it when I removed the old. Yep I'm a club member. V V V __________________ 1985 C10 shortbed, current project. Member of the Upstate SC GM Truck Club.

12-02-2018, 02:11 PM	#4
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Odd brake issue Is this you Harry? __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

12-02-2018, 03:19 PM	#5
72LB Registered User Join Date: May 2017 Location: Inman/Boiling Springs, SC Posts: 420	Re: Odd brake issue yep. __________________ 1985 C10 shortbed, current project. Member of the Upstate SC GM Truck Club.

12-02-2018, 09:15 PM	#6
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Odd brake issue How are the rear shoes. Are they tight on the hub when you install it? If not you need to adjust them to just barely rub to make them work.This needs to be done before bleeding. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

12-02-2018, 10:18 PM	#7
72LB Registered User Join Date: May 2017 Location: Inman/Boiling Springs, SC Posts: 420	Re: Odd brake issue Did that. __________________ 1985 C10 shortbed, current project. Member of the Upstate SC GM Truck Club.

12-03-2018, 07:03 AM	#8
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Odd brake issue Well as we spoke on the club page I would get a new prop, valve and start over. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

12-03-2018, 11:31 AM	#9
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Odd brake issue In case you haven't fixed the problem: I had a similar problem on my truck and it turned out to be new defective brake master cylinder -actually two from same parts store. I finally got my money back and got a different brand. The difference between you're problem an mine is that my pedal would go down to the floor but not really fast. Turns out the master cylinder piston was allowing fluid to go past it. I'm sure you already know this but for others that may not know, it's very important that you bench bleed the master cylinder according to the directions. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive... Last edited by 68gmsee; 12-03-2018 at 11:52 AM.

12-03-2018, 08:33 PM	#10
72LB Registered User Join Date: May 2017 Location: Inman/Boiling Springs, SC Posts: 420	Re: Odd brake issue Andy from what I have found a bad prop valve won't do what this one is doing. 68gmsee from what I have found it can be one of two problems, a bad booster or master. Changed out the booster this afternoon and no change so MC is next tomorrow. __________________ 1985 C10 shortbed, current project. Member of the Upstate SC GM Truck Club.

12-04-2018, 04:20 PM	#11
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Odd brake issue Hope the mc does it for you. If possible, try a different brand. Other than lots of air in the lines and or really bad leak, there's not much else. Keep us posted on the fix. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

12-04-2018, 05:28 PM	#12
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,426	Re: Odd brake issue >>with the truck off I have a hard pedal<< That pretty much rules out bleeding, shoe adj or Metering valve or Proportion valve, unless you had to pump the pedal to get that hard pedal. >> but when I start the truck brakes go to the floor.<< As it should, to a point. A correct size and operating booster should allow three strokes of the pedal should the engine die and loss of manifold vacuum. With the engine OFF, brake pedal depressed enough times to release all vacuum - while holding pedal down, start the engine. The pedal SHOULD drop. If the pedal goes all the way to the floor, the booster is working, the MC is working. but you DO have a problem. It sounds like one of two possible problems. Either the MC piston diameter is too small or the pedal-to-booster rod is mounted too high on the pedal arm. __________________ '67 GMC 2500, 292, 4spd, AC



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